Status update on the presence of pseudo-mirrors and non-isolated mirrors in IJmuiden-ringed adult European Herring Gulls

In a blog post on birdingfrontiers.com in 2013, Peter Adriaens offered a new identification feature for distinguishing an adult American Herring Gull from a European Herring Gull: the presence of a “grey mirror” (also referred to as a ‘pseudo-mirror’) on the underside of primary P9 and/or P10.

It appears as a small, light area located in the black section in the inner web of the feather, and is fully surrounded (isolated) by the black. It varies in shape and size.

Also discussed was a very similar feature: the ‘grey cut’ on the underside of P9 and/or P10. Instead of being fully surrounded, it is located along the feather edge and therefore open on one side (not isolated). It is also located in the inner web and also varies in shape and size.

Figure: An example of grey cuts in the inner web of P10 (left) and pseudo-mirrors in the inner web of P9 (right) in adult European Herring Gulls.

The article immediately raised my interest and made me check my local European Herring Gulls in an effort to confirm the presence or non-presence of these features.

In my case, I was able to check these features in colour-ringed adult European Herring Gulls of the IJmuiden Forteiland gull colony, the Netherlands, primarily by collecting data over three subsequent seasons (2015, 2016 and 2017).

I followed multiple individuals over these multiple seasons, and compared the existence of these features for each season in each of these individuals.

I believe that I have gathered enough data for a first analysis, which I will describe in this blog post.

Note: This article was the starting point of a more extensive, general article written together with Mars Muusse and made available on gull-research.org: Using pseudo-mirrors as an identification feature for adult Herring Gulls.

For a Dutch version of this article, see Status update op de aanwezigheid van pseudo-spiegels en grijze inkepingen bij adulte Zilvermeeuwen van IJmuiden Forteiland.

This article is also available on ResearchGate.

Summary

To start with a summary of my findings:

Pseudo-mirrors in adult European Herring Gulls are not common but not rare either
Grey cuts in adult European Herring Gulls are fairly common
The presence, shape and size of pseudo-mirrors or grey cuts in an individual adult European Herring Gull vary each moult cycle

Examples

Below follows an overview of the birds that I have come across so far. As mentioned, all birds are part of the research project that takes place in the IJmuiden Forteiland colony, and can be individually recognized by a green colour ring with a 4-letter code starting with ‘Y’.

All photos were taken by me in the gull colony of IJmuiden Forteiland, the Netherlands, unless otherwise stated.

Individuals with a pseudo-mirror

Four individuals have been recorded so far. Although this is a very low number, I think there is an explanation for why this is which I will address in the discussion at the end of this article.

Female YBAH showed a pseudo-mirror on the underside of P10 in the left wing in April 2015 (age older than 7 calendar years).

Male YBAJ showed a pseudo-mirror on the underside of P9 in the right wing in April 2015 (age older than 10 calendar years).

Male YBKC showed a pseudo-mirror on the under side of P8 and very likely one on P9 as well in April 2015 (age older than 6 calendar years).

Male YCAD showed two pseudo-mirrors on the underside of P9 in the left wing and in the right wing in May 2017 (age older than 4 calendar years).

Individuals with a grey cut

Adult YADH showed two grey cuts on the underside of P10 in the right wing in May 2016 (age older than 9 calendar years). It also showed a thayeri-pattern on P9 in the left wing.

Female YAHA showed an almost complete grey cut on the underside of P10 in both wings in June 2010 (age older than 6 calendar years). Location: Wijk aan Zee beach, the Netherlands.

Discussion

The main outcome of my research so far is that I have been able to record the presence of pseudo-mirrors in various individual European Herring Gulls. This indicates that this feature cannot be attributed to American Herring Gull alone.

In fact, in previous blog posts about this topic, I refer to various other examples in ringed and non-ringed individuals, found by myself and by others in Europe.

The reason why this has not been noticed, or at least has not been given much attention, is something that I think can be explained.

If we look at the ringed individuals in my examples across multiple seasons namely, we see that the pattern that is shown in one moult cycle (for example the presence of isolated or non-isolated mirrors), can be much different from that in previous or subsequent moult cycles.

In other words: the same individual can show pseudo-mirrors or grey cuts in one season, but not in another.

Some examples:

Female YBAH showed a pseudo-mirror on the underside of P10 in the left wing in April 2015. In the following moult cycle, it was not present (photo April 2016):

Male YBAJ showed a pseudo-mirror on the underside of P9 in the right wing in April 2015. This was not present in 2016 (May) and 2017 (May):

Male YBKC showed a pseudo-mirror on the under side of P8 and very likely one on P9 as well in April 2015.

In the previous moult cycle (2014) the primary pattern of YBKC showed no such gray pseudo mirrors, although a tiny indentation could be seen along the edge of P10 in the left wing.

Other differences in the primary pattern can also be seen, such as the size of the mirror on P9 (much larger in 2014 than in 2015).

(Photo June 2014)

In 2016 and 2017, no pseudo-mirrors or grey cuts were present, but again the size of the mirror on P9 is much larger than in 2015 (photos April 2016 and June 2017):

Female YBAZ showed almost complete grey cuts on the underside of P10 in both wings in May 2016, although the cut was also open at the top (age older than 8 calendar years).

In the following moult cycle of 2017, there is no indication of this feature (photo April):

So when it comes to scoring pseudo mirrors or grey cuts in adult Herring Gulls, it very much depends in which moult cycle you come across an individual. This makes it very hard to find such characteristics and preferably requires being able to follow known (ringed) individuals over multiple seasons.

I also belief that features like these stay under the radar because of a lack of knowledge among ringers in general, and gull ringers in particular. Plumage characteristics are often not photographed and in those cases where photos are taken of an open wing, it is usually of the upper wing.

I would strongly advice ringers of large gulls to start documenting the under wing and to do this for both wings.

More research is welcome

I will obviously continue monitoring the colour-ringed Herring Gulls of the IJmuiden gull colony and keep documenting the changes and variation in as many individuals as possible over as many seasons as possible.

I would like to emphasize to photographers the importance of photographing ringed gulls in general and with open wings in particular and to include those photos when submitting observations.

As Peter Adriaens mentions in his article, there is still much to discover when it comes to plumage characteristics of primary patterns in gulls and citizen science can definitely play a big role in accumulating this knowledge.

For more information about this subject, see the article that I wrote together with Mars Muusse on Gull Research: Using pseudo-mirrors as an identification feature for adult Herring Gulls.

Acknowledgements

I would like to thank Fred Cottaar and José Verbeek-Cottaar for allowing me to be part of their research team and for their enjoyable company and discussions about gulls, PBN and IJmuiden Forteiland for allowing us access to their site and for letting us study the breeding gulls, and Dr Kees Camphuysen Senior Scientist NIOZ for managing and guiding the research project and for providing us with those beautiful green colour rings.